Dynamics of molecular crystals

This chapter illustrates the dynamics of molecular crystals. The intramolecular vibrations and intermolecular vibrations (usually called lattice vibrations) are discussed. Because the chapter deals with molecules, two types of lattice vibrations can be distinguished; translational and rotational. In order to describe these motions, the potential energy of the crystal is expressed as a function of the center of mass positions and the orientations of all molecules. The chapter provides a detailed description of the different ways in which the potential can be expressed, each way having its own merits, depending on the subsequent calculations in which it has to be used. Later, the chapter briefly discusses the harmonic and quasi-harmonic models that are commonly used to describe the molecular motions— that is, the lattice vibrations, in molecular crystals. A description of perturbation theory around the harmonic model and of the self-consistent phonon method is also presented. The dynamical models for large-amplitude motions are also presented.